I have enabled KF8 mobi output in Calibre. I noticed that in "job details" both mobi6 and kf8 are generated. But it turned out that there's only one mobi file. (I was converting an epub book to mobi). Maybe the second one just replaced the first one. Any ideas?

It's working like it's supposed to... both the MOBI6 version and the KF8 version are contained in one file (two MOBI headers). Just like kindlegen produces. A device/app capable of reading KF8 will read the KF8 portion and a device that's incapable will read the MOBI portion. If you wanted to make two separate stand-alone files... just forget the combined_format tweak and convert twice: once to MOBI and once to AZW3.

By the way, if I choose to convert a file to AZW3, does it mean I get KF8? (And the file would be smaller than what I get right now)

Yes. For all practical purposes, an AZW3 is a stand-alone KF8 file... and it will definitely be smaller than the combined mobi/KF8 file (using the same input file, of course). Just be aware that not all devices/apps will be able to read that AZW3 file.

Yes. For all practical purposes, an AZW3 is a stand-alone KF8 file... and it will definitely be smaller than the combined mobi/KF8 file (using the same input file, of course). Just be aware that not all devices/apps will be able to read that AZW3 file.

Question about this, I'm been trying out Calibre's AZW3 format for use on my Kindle (NT) and it cant' seem to open the book. I have the latest firmware for the 4NT (4.1) which supports KF8 so i'm wondering why I'm unable to open a book in azw3 format?